Sen. Arlen Specter of Pennsylvania provided a boost to President Obama's ambitious legislative agenda yesterday by abandoning the Republican Party in the face of shifting political realities at home and an aggressive courtship by the White House and party leaders.

In an announcement that shocked colleagues on both sides of the aisle, Specter said he had become increasingly uncomfortable as a moderate in a party dominated by conservatives and would join the Democrats. He bluntly admitted that his decision was tied to his belief that he could not win reelection as a Republican next year.

Although he said he "will not be an automatic 60th vote" for Democrats, Specter's decision left Democratic Party leaders jubilant. The addition of Specter to their ranks, coupled with the likelihood that the Minnesota Supreme Court will name Al Franken the winner of that state's disputed Senate race in the coming months, means that Democrats are all but certain to control a filibuster-proof 60-seat majority in the chamber for the first time in about 30 years.

The news came on the eve of Obama's 100th day in office, and in a phone call shortly after he was informed of the party switch, the president promised Specter his "full support" in attempting to secure another term in 2010. Specter will appear with Obama and Vice President Biden, who helped lead efforts to bring Specter into the party fold, at the White House this morning.

Neither party has controlled 60 or more seats since 1978, and Republicans warned yesterday that such a majority would give Obama almost unfettered control over the federal government. But Specter vowed to maintain his current policy positions -- including opposition to a labor organizing bill and to the nominee Obama has tapped to run the key legal counsel unit at the Justice Department.

But even as Specter pledged his continued independence, Senate Majority Leader Harry M. Reid (D-Nev.) later told reporters that there is an effort underway to refashion the union legislation in an effort to gain Specter's support. The Pennsylvanian supported the legislation, known as the Employee Free Choice Act, in 2007 but announced his opposition to the bill in its current form as his primary challenge from former congressman Pat Toomey (R) gained momentum.

Democrats also hope the move will increase momentum for Obama's universal health-care plan, given that Specter, who has battled a brain tumor and Hodgkin's disease, is an ardent advocate of increased spending on medical research and causes.

Specter, 79, will retain seniority in the Senate as if he were elected as a Democrat when he first took office in 1980. As a result, he will likely receive a plum subcommittee chairmanship on the Appropriations Committee in the future, and he indicated that his goal is to one day chair the full committee. He has already served as Judiciary Committee chairman, after conservatives put aside major ideological concerns to elevate him to that post in 2004, and until yesterday he was the top Republican on that panel.

The decision was the culmination of a months-long effort by key Democrats to woo Specter, who began his political career as a Democrat in Philadelphia but has been a Republican for 43 years. Biden, a regular Amtrak passenger with Specter as the two traveled to Wilmington and Philadelphia, respectively, when both served in the Senate, met with him face to face six times and spoke on the phone with him on eight more occasions since mid-February, aides said. Pennsylvania Gov. Edward G. Rendell, whose first job as a prosecutor in Philadelphia came under the tutelage of then-District Attorney Specter, had also lobbied him about making the switch, but it was his Senate colleagues who apparently closed the deal.

Senate Majority Whip Richard J. Durbin (D-Ill.) said he approached Specter at the Senate gym a few weeks ago and, aware that he faced a difficult primary next year, asked, "Did you ever think about returning to your original party?" He said Specter responded, "You know, a number of people have talked to me about that."

During a vote Monday evening, Durbin realized the prospect of a switch could be more serious when he saw Specter's wife, Joan, seated in the visitors gallery. Specter and Reid then disappeared into the majority leader's office, and Durbin got a call from Reid a short while later, telling him the deal was done.

Specter's political standing in Pennsylvania has become increasingly tenuous in recent years. His record as a moderate, combined with the shrinking GOP base in the Keystone State, would make a general election difficult, and Toomey, who came within two percentage points of defeating Specter in 2004, was leading in public polls by double digits heading into next April's GOP primary.

Specter received his own final poll Friday, an assessment he called "bleak." He ultimately chose to cast his lot with Democrats, he said in a news conference yesterday, because "I am not prepared to have my 29-year record in the United States Senate decided by the Pennsylvania Republican primary electorate."

A handful of Pennsylvania Democrats had been considering pursuit of the Senate nomination, but potential opposition to Specter began to melt yesterday as the would-be contenders learned that he would have support from Obama and practically every leading Democrat in Washington.

Earlier this year, Specter outraged his Republican colleagues by supporting Obama's $787 billion economic stimulus program. Specter said at the time that the plan -- which he worked with two other Republicans to trim by more than $100 billion -- was necessary to avert another Great Depression. Toomey jumped in the race after he cast the votes, and Democrats soon stepped up their courtship efforts.

"The stimulus vote was a schism," Specter told reporters yesterday.

A decade ago, Republicans counted nine senators from the 11 states stretching up the Interstate 95 corridor north of the Capitol; today, they have three GOP senators from those states, and one, Judd Gregg (R-N.H.), will retire in 2010.

While Democrats celebrated the surprise move, Republicans alternately blasted Specter as a turncoat who had embraced political expediency over principle, or sank into soul-searching about the future of their party. Sen. Olympia Snowe (R-Maine), a fellow Northeastern moderate, called the news "devastating" for a party that has been unable to appeal to centrist voters.

Senate Republican leaders appeared ashen after Specter made a brief appearance at their weekly policy luncheon to tell them the news in person. "Obviously, we are not happy that Senator Specter has decided to become a Democrat," Senate Minority Leader Mitch McConnell (Ky.) told reporters, attempting to minimize the blow. "This is not a national story. This is a Pennsylvania story," he said.